My question is, why is that pressure only on the back side of the intake valve?

If you have say 15psi, it's filling the cylinder through the entire intake stroke and as the piston is coming up from bottom dead center chasing the intake valve closed, would it not be fair to say that boost pressure and pressure in the cylinder has equalized (or close) at that point?

Then as soon as the intake valve touches down you're on the compression stroke, far exceeding any boost pressure.